Adsorption-induced reconstruction

In this section we will discuss main features of atoms and molecules chemisorbed to transition-metal SLufaces. Adsorption induced reconstruction will be included. 

So far, it has been accepted that molecules or atoms are adsorbed on sites being in registry with the substrate lattice, and the catalysis is explained by their reaction over the surface. By the adsorption of molecules or atoms, surface metal atom undergoes so often short distance shift from their equilibrium position, which is the adsorption induced reconstruction. It should be pointed out that such short distance shift of the... 

Adsorption-induced reconstruction, surface metal atom, 245 Adsorption isotherms, development, 4/,5 Alcohols, role in photooxidation reaction of water on n-Ti02 electrode, 297-308 Alloy single-crystal surface, thin anodic oxide overlayers, 236-244 Alloy surfaces, chemical reconstruction, 246-247... 

R. Feidenhansl, 1. Stensgaard, Oxygen-adsorption induced reconstruction of Cu(llO) studied by high-energy ion-scattering. Surf. Sci. 133(2-3), 453-468 (1983)... 

The OH Cu ratio is 1 4 with respect to the reconstructed Cur plane and 9 49 with respect to the unreconstructed Cu plane. Thus about every five Cu atom has to be excluded from the topmost plane by the adsorption-induced reconstruction and is relocated at step edges or clustered on the terraces. The coverage of 9 49 (0.18 ML) is in agreement with the charge density of 55 pC cm of the anodic and cathodic peaks (a) and (c) in Figure 5.51a. Indeed, this charge corresponds to 3.4 x 10 OH cm" according to Equation 5.38, which is 19% of 1.75 X 10 Cu cm of the unreconstructed Cu(lll) planes. 

Scheme of OH adsorption-induced reconstruction of Cu(OOl) with formation of the Cu-OH-Cu trimers and selfarrangement in zigzag lines and c(2 x 6) superstructure. (From Kunze, J. et at,/. Electroanal. Chem., 554,113,2003.)... 

Perhaps the most fascinating detail is the surface reconstruction that occurs with CO adsorption (see Refs. 311 and 312 for more general discussions of chemisorption-induced reconstructions of metal surfaces). As shown in Fig. XVI-8, for example, the Pt(lOO) bare surface reconstructs itself to a hexagonal pattern, but on CO adsorption this reconstruction is lifted [306] CO adsorption on Pd( 110) reconstructs the surface to a missing-row pattern [309]. These reconstructions are reversible and as a result, oscillatory behavior can be observed. Returning to the Pt(lOO) case, as CO is adsorbed patches of the simple 1 x 1 structure (the structure of an undistorted (100) face) form. Oxygen adsorbs on any bare 1 x 1 spots, reacts with adjacent CO to remove it as CO2, and at a certain point, the surface reverts to toe hexagonal stmcture. The presumed sequence of events is shown in Fig. XVIII-28. 

Pettinger B, Lipkowski J and Mirwald S 1995 In situ SFIG studies of adsorption induced surface reconstruction of Au (111 )-electrodes Electrochim. Acta 40 133-42... 

The model is intrinsically irreversible. It is assumed that both dissociation of the dimer and reaction between a pair of adjacent species of different type are instantaneous. The ZGB model basically retains the adsorption-desorption selectivity rules of the Langmuir-Hinshelwood mechanism, it has no energy parameters, and the only independent parameter is Fa. Obviously, these crude assumptions imply that, for example, diffusion of adsorbed species is neglected, desorption of the reactants is not considered, lateral interactions are ignored, adsorbate-induced reconstructions of the surface are not considered, etc. Efforts to overcome these shortcomings will be briefly discussed below. 

According to the data obtained with SXRS in salt solutions,519 520 at a < 0 the surface of Au(lll) forms a ( 3 x 22) structure as in a vacuum. At a > 0 the reconstruction disappears and the (1 x 1) structure is observed. On the reconstructed Au(l 11) surface there are 4.4% more atoms than on the (1 x 1) structure and on the reconstructed Au( 100) there are 24% more atoms than on the (1 x 1) structure.506,519 This phase transition shifts in the negative direction with the adsorbability of the anion. The adsorption-induced surface reconstruction of Au(l 11) electrodes has been studied in situ by second harmonic generation by Pettinger et al.521... 

The changes in local coordination of the majority specie (the substrate) at the surface (e.g. adsorbate induced reconstruction). It all depends on the detection mode and on the absolute surface sensitivity of the most surface sensitive detection mode. The way to proceed is to measure differential SEXAFS spectra on one adsorption edge of the substrate material, where the difference is made between the clean substrate and the exposed substrate, or between a bulk sensitive detection of the EXAFS, (TY, FY, high energy AEY) and a surface sensitive measure (low energy AEY). Attempts have been made on this groxmd, but final reports have not yet appeared in the literature. 

The most important features of both the reflectance and the photolumi-nescence spectra have been explained by the preceding model since it is based on ideal surface structures essentially determined by (001) planes. Thus, several likely possibilities, such as the presence of surface defects, impurities, and remaining adsorbates, the relaxation of the planes exposed at the surface, the impurity-induced reconstruction of the surfaces, and changes in the force constants, have been excluded (80). A more detailed model is needed in which the ion pair of the metal cation and oxygen anion can be taken into account on the basis of such experimental evidence as the hydrogen adsorption on MgO obtained by Coluccia and Tench (65) and Ito et al. (90). 

Jenkins SJ (2006) Dissociative adsorption and adsorbate-induced reconstruction on Ee 211. Surf Sci 600 1431... 

C. Baldacchini, M. G. Betti, V. Cor-radini, and C. Mariani, Au(l 10) induced reconstruction by 7t conjugated molecules adsorption investigated by photoemission spectroscopy and low energy electron diffraction. Surf. Sci. 566, 79-83 (2004). 

A method to elude those defects, induced reconstructions, or anion adsorption is to transfer the electrodes under well-controlled conditions including atmosphere. Thus, undesirable effects from oxygen adsorption or impurities as a source of voltammogram modifications can be avoided. These requirements are fulfilled in the iodine-carbon monoxide substitution method which was proposed for the preparation of clean and well-ordered Pt( 111) [66] and applied to Pt(100) clean surface preparation [67]. An interesting alternative to this method would be to find experimental conditions that maintain a carbon monoxide adlayer for surface protection during the transfer, assuming that this adsorption is innocuous for the surface structure itself. If this efficient protection makes no detectable surface-order modifications for Pt(100) electrodes as deduced from the cyclic voltammetric contour, we can conclude that this protection method is convenient for studying the influence of anion adsorption on the surface structure in transfer experiments. 

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